Capacitive signal detectors are conventionally used to assess the performance of ignition systems, such as coil-on-plug (COP) systems utilizing one coil per cylinder or a direct ignition system (DIS) or double ended coil-on plug (DECOP) utilizing one coil per cylinder pair. Capacitive signal detectors can output signals indicative of spark plug firing voltage and duration, which help technicians determine if any component in the ignition system is malfunctioning. Such conventional systems may be found in, for example, U.S. Pat. No. 4,399,407 to Kling et al., U.S. Pat. No. 5,461,316 to Maruyama et al., U.S. Pat. No. 5,677,632 to Meeker, and U.S. Pat. No. 6,396,278 to Makhija.
Almost all capacity adapters, whether for sampling coils or ignition wires, employ a capacity divider. As shown in FIG. 1(a), one side of the capacity divider comprises a very small capacitor C1, one side of which is placed in the vicinity of the high voltage electric field present during the ignition system spark cycle. The other side of the capacity divider is connected to the top of a much larger capacitor C2 whose bottom side is connected to ground. The ratio of the values of these two capacitors, the high voltage division ratio, is typically between about 5000:1 and 10,000:1, which corresponds to a voltage across the larger capacitor of between about 10V and 5V for a 50 kV firing line. Under ideal conditions, the ratio is independent of frequency so that the waveform (e.g., the firing line, spark line, or spark duration) is not distorted.
FIG. 1(b) shows an equivalent circuit for the equipment configuration represented in FIG. 1(a). Voltage E1 represents the voltage at the sensor 10 proximally coupled to the COP ignition 30 and voltage E0 represents the output voltage. The ratio E0/E1 is equal to the ratio (I*R2)/(I*(R1+R2) or, simplifying, R2/(R1+R2). When R1 is much greater than R2, the ratio E0/E1 further simplifies to R2/R1. By superposition, when XC1 is substituted for R1 and XC2 for R2, the ratio E0/E1 further simplifies to XC2/XC1. This is a compensated, or frequency insensitive, divider since E0/E1 is the same for resistances (R's) and capacitive reactances (XC's).
Accordingly, conventional adapters are dedicated or designed, developed, and fabricated for specific combinations of a COPs and a display device or lab scope. In other words, these specifically configured adapters are balanced for use with a particular combination of coil and equipment. However, if any component in the combination is disturbed by substituting a non-dedicated component for a dedicated component in a balanced system (e.g., a different lab scope is used), the system is not longer compensated or independent of frequency. Accordingly, the resulting waveforms conveying data on the firing line, spark line, or spark duration are undesirably distorted. An example of this is shown in FIG. 1(c), wherein the spark kV or spark line voltage decreases notably with time prior to the return to zero instead of remaining reasonably or substantially constant.
For example, a resistance value R2 for a first engine analyzer or lab scope may be 1 MΩ, whereas a resistance value R2 for a second engine analyzer or lab scope may be 10 MΩ. Thus, a sensor system balanced for use with the first scope will not be balanced for use with the second scope.
Therefore, there is a need for a compensated capacity divider which may be adjusted to be frequency independent or insensitive for many different combinations of capacitive sensors and diagnostic equipment or lab scopes.